With enhancement of packaging density of electronic equipment in recent years, progress has also been made in layer multiplication of the printed wiring boards used in such electronic equipment and use of the flexible wiring boards of a multilayer structure is gaining ground. The printed wiring board is a rigid-flexible wiring board which is a composite of a flexible wiring board and a rigid wiring board, and the range of its use is expanding.
The following methods have been proposed for producing the multilayer flexible wiring boards or rigid-flexible wiring boards: The patterned copper foils and the insulating layers are placed one on the other alternately to form a laminate, then the through-holes for interlaminar connection are formed in the said laminate, and after applying plating for interlaminar connection on the said through-holes, working of the outermost layer circuits, etc., is conducted; On the insulator side of each single-sided wiring board are formed the holes which do not penetrate the copper foil, then conductor posts are formed with a metal or an alloy, followed by surface coating of the whole layers, and an adhesive layer and each wiring board are press bonded, with the above operations being repeated a necessary number of times for effectuating the desired layer multiplication. (See, for example, JP-A-11-54934.)
In the former method, interlaminar connection is made by forming the through-holes in such a way that they pierce through all the layers. However, this connecting method, although simple in working, is subject to many restrictions on circuit designing. The greatest drawback of this method is that since all the layers are connected by through-holes, the through-holes formed in the outermost layers are increased in number and also the areal ratio occupied by the through-hole lands elevates, making it unable to raise the circuit density which is essential for dense packaging of parts and circuit patterning. Also, high-density packaging and patterning, for which the market demand is expected to rise in the future, would become works of more complex specifications. With further advance of miniaturization and densification of the packaged parts, the layer connecting lands and through-holes need to be formed at a same location through the layers, so that the wiring density becomes deficient in design, giving rise to the problems in packaging of the parts.
In the conventional production methods of the flexible wiring boards, in order to reduce the production cost, they are manufactured by arranging plural patterns on a single sheet. It is possible to produce the multilayer flexible wiring boards, too, at low cost by using the same method. According to this method, however, if any defective pattern exists in the sheet, the multilayer flexible wiring board having such a defective pattern in the laminate also become defective, causing a reduction of process yield in the laminating process. The greatest difference between the multilayer flexible or rigid-flexible wiring board and the multilayer rigid wiring board is that the former has a flexible portion while the latter does not. In construction of this flexible portion, it needs to eliminate the outer layers so that the flexible portion won't be laminated or to remove the outer layers after lamination, so that a drop of material yield is inevitable in sheet lamination. Further, in the case of designing the different sizes of patterns for the respective layers, the number of the patterns that can be formed per sheet is restricted by the number of the maximum size patterns, resulting in an unsatisfactory pattering ratio and a low material yield.
In the latter production method, there are included the steps of specific operations for boring the substrate on the conductor post receiving side by laser, applying a desmearing treatment and forming openings in the surface coating, so that there exist the problems on the establishment of techniques for these specific operations and reduction of yield. There also is the problem that as the number of the structural layers increases, a longer time and higher cost are necessitated for the production and further the material cost for the surface coating elevates.